This invention was made under Subcontract No. ZAX-8-17647-10, which is a subcontract under U.S. Prime Contract No. DE-AC36-83CH10093.
1. Field of the Invention
This invention relates to the use of lasers for cutting semiconductor materials and more particularly to the laser cutting of EFG-grown material.
2. Background of the invention
The EFG method of growing shaped crystalline materials from a melt, is well known, as exemplified by U.S. Pat. Nos. 4,937,053, 5,102,494 and 5,558,712. The use of EFG materials in fabricating photovoltaic cells also is well known, as exemplified by U.S. Pat. Nos. 4,751,191, 5,698,451, 5,106,763 and 5,151,377. It has been the practice to use the EFG method to grow hollow polygonal bodies of a doped semi-conductor material, e.g., octagons of phosphorous-doped silicon, and to use a laser to subdivide those bodies into rectangular wafers or blanks for use in making photovoltaic cells. More recent efforts have been directed to growing large diameter, thin wall cylinders of silicon, e.g., cylinders with a diameter of approximately 50 cm and a wall thickness of 100 to 700 xcexcm (microns).
Laser cutting of a material involves absorption of laser energy within the material, resulting in a localized temperature increase, melting and vaporizing of the material and transport of the melted and vaporized material away from the body being cut. The development of existing technology for cutting wafers out of hollow EFG-grown silicon bodies has been driven by the need to maximize cutting speed, which has lead to a search for lasers with the highest available average power. Heretofore the cutting up of hollow silicon bodies into wafers has been accomplished in air using conventional industrial lasers such as conventional Nd:YAG and CO2 lasers with average power levels of about 100-500 watts (xe2x80x9cWxe2x80x9d). Those lasers are similar in their cutting action in that they are operated with relatively long pulse lengths, about 300 and 500 microseconds (xcexcs) respectively, and the cutting proceeds with a predominantly melting mechanism. These prior methods of laser cutting silicon bodies have cutting speeds in the order of 25-50 mm/sec. for 300 xcexcm thick silicon. Moreover, the same prior methods of laser cutting the brittle EFG-grown silicon bodies result in laser-induced damage to the edges of the silicon wafers.
Laser-induced edge damage to the silicon wafers is the result of changes in the physical material properties where heating has occurred. The material depth to which these changes are detectable is called the heat-affected zone (xe2x80x9cHAZxe2x80x9d). The defects in the HAZ are of two typesxe2x80x94microcracks and deposited debris (xe2x80x9cslagxe2x80x9d). The microcracks, which usually are numerous, commence at and generally propagate normal to the cut edges and then turn by about 90 degrees so as to extend parallel to the cut edges. By way of illustration, when cutting silicon in air with a thickness of 300 xcexcm using a Nd:YAG laser with a pulse duration of 0.5 milliseconds (msec), a pulse energy of 500 millijoules (mJ) and a Gaussian spot with a beam diameter of approximately 150-200 xcexcm, the microcracks may extend up to 50 xcexcm away from the cut edges. These microcracks are highly undesirable since they result in mechanical weakness of the laser cut wafers. The slag consists primarily of silicon which has resolidified from the molten and volatilized states and SiO2 (when air is present in the cutting zone). The deposited slag contributes to the weakening of the edges of the cut wafers.
The damaged edge material needs to be removed by chemical etching before the cut wafers can be processed to manufacture photovoltaic cells. The deeper the HAZ, the more material must be removed at the edges of the wafers. Unfortunately the occurrence of edge damage has resulted in a reduction in the yield of acceptable wafers for solar cell fabrication purposes and also in increased costs due to the need to remove damaged edge material from the wafers before commencing the several processes required to convert the wafers to solar cells or other devices.
The critical laser characteristics influencing the laser beam interaction with crystalline silicon and the extent of the HAZ are laser power and the variations of the absorption coefficient with wavelength, temperature, peak laser power, and gas composition. These factors impact on the cutting speed limits as well as on the mechanism of heat transport and subsequent HAZ formation and edge damage generation. However, in striving to reduce edge damage, the basic requirement of maximizing laser power so as to increase the cutting speed must be coupled with limiting the heat input to the silicon material, since the heat input to the material surrounding the laser cut determines the extent of the HAZ and the occurrence and length of the microcracks. Depending on the laser beam wavelength, the pulse length must be sufficient so that enough energy is initially retained in the silicon crystalline material to heat it above about 600xc2x0 C. to attain melting of the silicon. The laser pulse energy must be maintained near the melting threshold to minimize the HAZ.
Damage free and rapid cutting of silicon using laser energy has remained an elusive but necessary goal in the effort to achieve high yields at the various stages in the process of converting the silicon wafers into photovoltaic cells or other semiconductor devices and to reduce manufacturing costs.
A primary object of this invention is to provide a new and improved method of cutting crystalline materials with a laser.
Another object of this invention is to provide a new and improved method of cutting semiconductor bodies with a laser so as to produce wafers for use in making solar cells or other solid state semiconductor devices.
A further object is to provide an improved method of cutting crystalline materials with a laser so as to reduce edge damage.
Still another object is to enhance the speed of laser cutting a crystalline material such as silicon.
A more specific object is to provide an improved method of cutting a thin silicon body with a laser at a rapid rate with minimal occurrence of micro-cracks or other damage at the edges of the cut material.
Another specific object is to provide an improved method of laser-cutting EFG-grown hollow silicon bodies into rectangular wafers or blanks, with the cutting being accomplished at optimum speed and with a reduced number of micro-cracks at the edges of the wafers.
The foregoing objects, and other objects that are rendered obvious by the following detailed description, are achieved by cutting semiconductor material with a laser in a selected a vacuum or a non-oxygen atmosphere. More particularly, the laser cutting is accomplished in a vacuum or in the presence of one or more gases from the group comprising forming gas and the noble gases (He, Ne, AR and Kr). Other features and advantages of the invention are disclosed in or rendered obvious by the following detailed specific description and the accompanying drawings.